Thermoset compositions such as epoxy resins are important in the manufacture and processing of circuit assemblies. The diverse number of applications include coatings, adhesives, structural materials, electrical insulation, as well as encapsulation and sealing. The attributes of epoxies include the combination of processability prior to curing with outstanding properties after curing. Epoxies generally have a low viscosity prior to curing, even in the absence of solvents. After curing, epoxies exhibit toughness, adhesion, and solvent resistance.
The attribute of epoxies also include intractability after curing. This intractability is the result of a curing reaction to convert a low molecular weight precursor to a polymer network of essentially infinite molecular weight. These attributes make epoxies ideal for use in the construction of circuit assemblies such as single-sided and double-sided circuits, as well as other types of surface mount technology including chip carriers, multichip modules and multilayer boards.
Examples of various polymer systems which are radiation curable include Day, U.S. Pat. Nos. 5,439,779 and 5,439,766, Smith, U.S. Pat. No. 4,256,828, Tortorello, U.S. Pat. No. 4,289,595, Knudsen, U.S. Pat. No. 5,366,846, Koleske, U.S. Pat. No. 5,155,143, and Eckberg, U.S. Pat. No. 4,987,158, which all describe UV curable compositions that incorporate onium salts, epoxy monomers, epoxy novolaks, silicone epoxides, and polybutadiene epoxides, in addition to resin modifiers such as hydroxy compounds, and inorganic fillers. Miller et al., U.S. Pat. No. 5,373,032 discloses radiation curable urethanyl prepolymers which polymerize in the presence of a cationic curing agent. Babich et al., U.S. Pat. No. 5,229,251 discloses an epoxide photoresist which is curable in ambient conditions when combined with an organo-silicon and an onium salt.
Fisher, U.S. Pat. No. 2,895,962 discloses acetal epoxides synthesized from the reaction of polyhydric alcohols and ethylenically unsaturated aldehydes.
However, these compositions, once cured, form non-reworkable and intractable masses. The intractability of thermosets has become more of a liability because of concerns about the longitivity of circuit assemblies in the environments of use. Also, many manufacturers are taking responsibility for disposal or recycling of their products. Manufacturers may even be required to be responsible for disposal or recycling of products through government regulation.
Intractable thermosets are also not compatible with the concept of disassembly for purposes of disposal, repair, or recycling, whether the thermosets are used as structural components, adhesives, or encapsulants. If, however, the thermoset itself is designed for disassembly on the molecular scale, it is possible that the many advantages of the thermosets can be retained without the disadvantages of intractability. As demand increases for recyclable products, diepoxide materials which allow for reworkability may well offer a means of maintaining the utility of thermoset materials which offer repair, replacement, recovery, or recycling.
As a result, there is a need for photosensitive encapsulants which provide the requisite curing properties and physical stability once cured which are at the same time reworkable so as to allow for the recovery of various thermosetting systems.